Reaction Mechanisms and The Rate Determining Step

Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

A reaction mechanism refers to the steps that occur in a chemical reaction. Learn about reaction mechanisms, including intermediates, and understand the role of the rate-determining step, a step that is slower than the other steps and controls the reaction rate. Updated: 08/25/2021

Reaction Mechanism

When you see a balanced chemical equation, it is tempting to think that the reaction takes place in one step. For example, in the reaction of A + B --> AB, it is easy to think that A and B just join and make AB in one step. But, this isn't necessarily true. It may take many steps to finally get to AB, and A and B may make an intermediate substance or two before they make the final product. When you write out the step-by-step sequence of reactions for a chemical reaction, it is called the reaction mechanism.

Let's look at the reaction of: H2 (g) + I2 (g) --> 2 HI (g). You might think that it is such a simple-looking reaction that there was only one step - an H combines with an I to make HI. This isn't what happens, though. Scientists aren't sure, but they believe that the actual reaction happens in either two or three steps.

Example A

  1. I2 --> 2 I
  2. 2 I + H2 --> 2 HI

The '2 I's cross out.

Example B

  1. I2 --> 2 I
  2. I + H2 --> H2I
  3. H2I + I --> 2 HI

Each 'I' in step one cancels out with an 'I' in step two and an 'I' in step three. The 'H2I's also cross out.

Notice that the 'I' in step one and the 'H2I' in step two aren't written in the net overall original equation. This is because they cancel out, just like in a math equation. The 'species,' such as the 'I's and 'H2I's, that appear in the steps but not in the net equation are called intermediates.

This reaction that we have been working with contains substances that are all in the gas phase. Even the intermediates of I and H2I are gases. Any reaction in which the reactants and products exist in a single phase is a homogeneous reaction.

An error occurred trying to load this video.

Try refreshing the page, or contact customer support.

Coming up next: State Functions in Thermochemistry

You're on a roll. Keep up the good work!

Take Quiz Watch Next Lesson
Your next lesson will play in 10 seconds
  • 0:05 Reaction Mechanism
  • 2:21 Rate-Determining Step
  • 3:48 Lesson Summary
Save Save Save

Want to watch this again later?

Log in or sign up to add this lesson to a Custom Course.

Log in or Sign up

Speed Speed

Rate-Determining Step

If one step is slower than the rest of the steps, then it is a rate-determining step. The rate-determining step controls the reaction rate. Think of what happens to a freeway when there is an accident closing all but one lane. The freeway can only flow as fast as the cars in that one lane can go. The one lane is the determining rate of flow.

To unlock this lesson you must be a Member.
Create your account

Register to view this lesson

Are you a student or a teacher?

Unlock Your Education

See for yourself why 30 million people use

Become a member and start learning now.
Become a Member  Back
What teachers are saying about
Try it now
Create an account to start this course today
Used by over 30 million students worldwide
Create an account